1. Field of the Invention
The field relates to a step-up/step-down type DC-DC converter, and a control circuit and control method of the same, more particularly, it relates to a step-up/step-down type DC-DC converter capable of reducing an average switching frequency of switching elements and of realizing high efficiency.
2. Description of Related Art
In a step-up/step-down type DC-DC converter, a state (1) for accumulating energy into an inductor from an input side and state (2) for discharging the energy from the inductor to an output side are alternately repeated at a predetermined frequency by on/off operations of switching elements, the inductor being connected to three terminals, a voltage input terminal, voltage output terminal and reference potential.
In a step-up/step-down type DC-DC converter disclosed in U.S. Pat. No. 6,087,816, there is further provided a state (3) for connecting a voltage input terminal to a voltage output terminal via an inductor, and supplying energy to an output. In one clock cycle, switching between the state (1) and state (3) or between the state (2) and state (3) is performed.
Furthermore, a conventional step-up type DC-DC converter 100 shown in FIG. 12 comprises transistors FET101 through FET103. When operation the DC-DC converter 100 is stopped, the transistor FET103 is set OFF so as to block dark current which flows from input voltage Vin to load RL.
Moreover, U.S. Pat. No. 6,275,016, Japanese Published Unexamined Patent Application No. 2005-192312, Japanese Published Unexamined Patent Application No. S55-68877, U.S. Pat. No. 5,402,060, U.S. Pat. No. 4,395,675, Japanese Published Unexamined Patent Application No. S56-141773 and Japanese Published Unexamined Patent Application No. 2000-134943 are disclosed as related art with the above.
Recent demands for miniaturizing and lightening electrical devices have advanced miniaturization of inductors. Which brings necessity to suppressing a peak-to-peak current value of inductor current, so that a switching frequency rises and a switching loss tends to increase. Therefore, it is necessary to reduce the switching frequency while suppressing the peak-to-peak current value of the inductor current to reduce the switching loss. However, reducing the switching frequency is not disclosed in U.S. Pat. No. 6,087,816, and the switching loss cannot be reduced, which pose a problem.
Furthermore, when the conventional step-up type DC-DC converter 100 shown in FIG. 12 starts up from operation-stopped state and the transistor FET103 is set ON, charge current to an output capacitor C101 turns into rush current Ir. Consequently, instantaneous lowering of input voltage occurs, which is problematic because it is likely to cause glitch such as invoking of a protection circuit, and the like. Furthermore, since an output voltage Vout sharply steps up to an input voltage Vin from 0 (V) due to rush current, that makes it impossible to carry out a soft-start control which is a control manner to step the up output voltage Vout from 0 (V) to a predetermined set voltage gradually. This is problematic aspect of the conventional art. Furthermore, rush current Ir can possibly destroy each constituent element of the circuit, which is also problematic.